I've used these people before but only via their shop in Alderly Edge - a bit far from Dorset! They offer excellent service and really know their stuff. They'll let you run up and down the street outside their shop 'til you find a pair that suits. They often have a stall at big running events:
http://www.runningbear.co.uk/
Never used these but this lot advertise regularly and seem to offer some good prices:
http://www.bournesports.com/